Corrosion processes of Mg-Y-Nd-Zr alloys in Na2SO4 electrolyte

The corrosion behavior of Mg-Y-Nd-Zr (WE43 commercial alloy) was investigated in Na2SO4 electrolyte using potentiodynamic polarization curves, X-Ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiles, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS) analyzes. SEM and EDS data show that Nd-rich precipitates are mainly located at the grains boundaries. Zr/Y-rich zones are distributed inside the most of the grains. XPS study indicates a depletion of Mg on surface that could be attributed to Mg dissolution and an enrichment of the addition element oxides. XPS and ToF-SIMS analyzes demonstrate that the corrosion films are made up of a magnesium hydroxide (Mg(OH)2) outer layer and an inner layer containing magnesium oxide (MgO), yttrium oxide (Y2O3) and hydroxide (Y(OH)3), mixed with a small amount of MgH2, zirconium oxide (ZrO2) and neodymium oxide (Nd2O3). The Y2O3 and Y(OH)3 signals increase slightly in the inner layer towards the corrosion film/alloy interface. Unlike these compounds, ZrO2 and Nd2O3 compound signals are constant inside the inner layer. It is concluded that: (i) neodymium, zirconium and yttrium play a key role in the slightly improved corrosion resistance of the alloy and (ii) the cathodic reaction is slower on WE43 than on pure Mg and AZ91.